Substrates containing polyphosphazene as matrices and substrates containing polyphosphazene with a microstructured surface

ABSTRACT

The invention relates to substrates containing polyphosphazene with a forming surface as matrices for producing biological materials that can be implanted in a mammal. The invention also relates to a method for producing said substrates and substrates containing polyphosphazene with a microstructured surface.

[0001] The present invention concerns substrates containing polyphosphazene with a forming surface as matrices for producing biological materials that can be implanted in a mammal, a process for producing such substrates, and substrates containing polyphosphazene with microstructured surfaces.

[0002] Culturing of cells, especially of endothelial cells, with the goal of growing artificial organs, is a comparatively new development in implantology. One particular advantage of this technology is that implants prepared in this manner are expected to exhibit complete compatibility with the body. Given that cell collections cultured ex vivo initially do not have either the shape or the mechanical stability desired for the later implants, such as organs, arteries, etc., such implants are initially preformed on a forming substrate. Examples of substrates currently used, on which such cells are cultured, and which are the primary supporting structure for such an implant, include polylactides, polyethylene glycols, polyurethanes, Teflon, and inorganic substrates.

[0003] Numerous other materials being used to produce such primary supporting structures or supporting substrates are known from the prior art and are being investigated. For example, an expandable shell of ε-PTFE, which can also be used for the culturing of artificial blood vessels, is known from WO 98/56312. Other materials for this application are described in EP-A-0 810 845, U.S. Pat. No. 4,883,699, and U.S. Pat. No. 4,911,691. Examples of other polymers for this purpose include hydrolyzed polyacrylonitrile (U.S. Pat. No. 4,480,642), hydrophilic polyethers (U.S. Pat. No. 4,798,876) and polyurethane diacrylates (U.S. Pat. No. 4,424,395). Also, various hydrogels are known that can be used as coatings for this purpose. The group of potentially applicable materials can also be supplemented by polyvinylpyrrolidone (PVP), polyvinyl alcohols (PVA), polyethylene oxide (PEO) and polyhydroxyethyl methacrylate p(HEMA). Furthermore, use of a group of standard materials such as polyurethanes, polyethylenes and polypropylenes is also described in the prior art as possible materials for such substrates. Mixtures of these materials with one another are also known. Another group of materials is known from EP-A-0 804 909.

[0004] Given that the inherent characteristics of these materials differ, it can be assumed that each of these materials, or each of these substances, exhibits special characteristics for certain applications in the culture of artificial implants. For instance, PVA dissolves very well in liquid and is rapidly absorbed. Other materials exhibit good compatibility with blood. Still other materials are particularly extensible. Unfortunately, however, all materials have drawbacks in various areas. For instance, PVA does not exhibit particularly good blood compatibility.

[0005] ε-PTFE, for instance, is quite extensible and also exhibits good blood compatibility; but this material is very difficult to handle and production of supporting substrates from this material requires a series of specific processing steps (see WO 96/00103). The surface of the ε-PTFE substrate obtained in that manner is also very porous, so that cells grow into this material very strongly and it is almost impossible to avoid damage when separating the cultured cell material for an implant from the supporting substrate. For other materials, elastic properties, which are important for such a supporting substrate in some cases, can be achieved only by adding plasticizers which reduce compatibility with the blood and body, and which also present an undesirable affect on the cell culture due to bleeding of the “plasticizers”.

[0006] The greatest difficulties that arise in the culturing of cells for implants are reactions with the supporting substrate or with its degradation products. It is known, for example, that inflammatory reactions can occur in recipients due to the dissolving or absorption and decomposition of some of the substances known in the state of the art (van der GieBen, Circulation, Volume 94, No. 7, 1996). Those arise either because of partially incomplete compatibility of such supporting substrates, or because of reaction with decomposition products that arise due to decomposition of the substances noted. Furthermore, cracks and fractures can occur in the freshly cultured implant when the cultured implant is to be removed from the supporting substrate. That disadvantageous effect is primarily due to the fact that the cells growing for the implant bind very tightly to the supporting substrate, particularly so with polylactide, for example; because of the pore structure that arises from dissolution or due to the basic surface nature of the supporting substrate, they intertwine with the supporting substrate so that it is practically impossible to remove them without damage.

[0007] Behavior with respect to bacteria and proteins that are deposited on the surfaces of the supporting substrate is also a major factor in the successful culturing of the implants noted, cells in particular, because these deposits can lead to significant inflammations in patients and to other problems with the growth and culture of the cells.

[0008] The cracks that have been mentioned, which can occur on removal of cultured blood vessel implants from the supporting substrate, are an important aspect in the production of vascular implants. These cracks are, for instance, points of attachment for increased development of thrombi in recipients or patients, and for other deposits (proteins, macrophages, etc.) that can become a risk for the recipients or patients after implantation.

[0009] Thus the present invention is based on the objective of providing a new system for producing implants from biological materials that is intended to allow the most selective growth possible of the desired cells and to assure essentially damage-free separation of implants made of the desired cells from the supporting substrate used.

[0010] This objective is achieved by the embodiments as characterized in the claims. In particular, the use of a substrate with a forming (or form building) surface is provided, comprising at least partially a biocompatible polymer with the following general formula (I)

[0011] in which n stands for 2 to ∞, R¹ to R⁶ are the same or different and stand for an alkoxy, alkylsulfonyl, dialkylamino or aryloxy group, or a heterocycloalkyl or heteroaryl group having nitrogen as the hetero atom, as a matrix for producing biological material that can be implanted in a mammal.

[0012] In one preferred embodiment of the present invention, the biocompatible polymer according to Formula (I) is provided as a coating on the substrate to develop the forming surface. In this embodiment of the invention, there is no particular limitation on the substrate used, and it can be any material, such as plastics, metals, metal alloys and ceramics. The biocompatible coating has, for example, a thickness from about 1 nm up to about 1000 μm, preferably up to about 10 μm, and especially preferably up to about 1 μm. In another preferred embodiment of the present invention the substrate having a forming surface is a shaped object or moulding or moulded article made of the biocompatible material according to Formula (I).

[0013] The degree of polymerization of the biocompatible polymer according to Formula (I) is preferably in a range of 20 to 200,000, more preferably from 40 to 100,000.

[0014] Preferably at least one of the groups R¹ to R⁶ in the polymer used is an alkoxy group, substituted with at least one fluorine atom.

[0015] The alkyl groups in the alkoxy, alkylsulfonyl and dialkylamino groups are, for example, straight-chain or branched-chain alkyl groups with 1 to 20 carbon atoms, wherein the alkyl groups can, for example, be substituted by at least one halogen atom, such as a fluorine atom.

[0016] Examples of alkoxy groups are methoxy, ethoxy, propoxy and butoxy groups, which can preferably be substituted by at least one fluorine atom. The 2,2,2-trifluoroethoxy group is particularly preferred. Examples of alkylsulfonyl groups include methylsulfonyl, ethylsulfonyl, propylsulfonyl and butylsulfonyl groups. Examples of dialkylamino groups include dimethylamino, diethylamino, dipropylamino and dibutylamino groups.

[0017] The aryl group in the aryloxy group is, for example, a compound with one or more aromatic ring systems, in which the aryl group can, for example, be substituted with at least one alkyl group as defined above. Examples of aryloxy groups are phenoxy and naphthoxy groups and derivatives thereof.

[0018] An example of the heterocycloalkyl group is a ring system containing 3 to 7 atoms, with at least one ring atom being a nitrogen atom. The heterocycloalkyl group can, for instance, be substituted with at least one alkyl group as defined above. Examples of heterocycloalkyl groups include piperidinyl, piperazinyl, pyrrolidinyl, and morpholinyl groups and their derivatives. The heteroaryl group is, for example, a compound with one or more aromatic ring systems, in which at least one ring atom is a nitrogen atom. The heteroaryl group can, for example, be substituted with at least one alkyl group as defined above. Examples of heteroaryl groups include pyrrolyl, pyridinyl, pyridinolyl, isoquinolinyl and quinolinyl groups and their derivatives.

[0019] In another preferred embodiment of the invention, a layer containing an adhesion promoter is placed between the surface of the substrate and the biocompatible coating made of the polyphosphazene derivative.

[0020] The adhesion promoter, or spacer, preferably contains a polar end group. Examples of those include hydroxy, carboxy, carboxyl, amino or nitro groups. End groups of the O-ED type can also be used, in which O-ED means an alkoxy, alkylsulfonyl, dialkylamino or aryloxy group, or a heterocycloalkyl or heteroaryl group having nitrogen as a hetero atom, which can be variously substituted, for instance by halogen atoms, especially fluorine.

[0021] In particular, the adhesion promoter can, for example, be an organosilicon compound, preferably an amino-terminated silane or one based on aminosilane, amino-terminated alkenes, nitro-terminated alkenes and silanes or an alkylphosphonic acid. Aminopropyltrimethoxysilane is particularly preferred.

[0022] The adhesion promoter in particular improves the adhesion of the coating to the surface of the substrate by coupling of the adhesion promoter to the surface of the substrate, through ionic and/or covalent bonds, for example, and by further coupling of the adhesion promoter to the described polymer of Formula (I) of the coating, for instance, through ionic and/or covalent bonds.

[0023] The term “biological material” includes, for example, eucaryotic cells, monolayer or multilayer cellular aggregations, tissues, or cell components of mammals, especially of human origin. In one preferred embodiment the donor of the biological starting material is identical to the recipient of the implantable biological material. Examples of the biological starting material or biological material include endothelial cells of various origins (e.g., from skin, foreskin, blood vessels such as the aorta, fatty tissues, eye, omentum, umbilical cord, varices, or the like), epithelial cells of various origins (e.g., from the stomach, intestine, or the like), bone cells, cartilage cells, and all adherent cells or cells in which adherence is inducible, cell aggregations or tissues (e.g., artificial cultured skin or similar tissue), natural tissues, proteins, sugar molecules and lipids. Artificial organs, blood vessels, bones, cartilage, myelin sheaths, etc., can be produced by using the substrate with a forming surface.

[0024] A further object of the present invention concerns a process for producing the substrates with forming surface as defined above; wherein the application of a coating of the biocompatible polymer according to Formula (I) to the surface of a shaping body, moulding (or moulded article) or supporting substrate is known from the prior art.

[0025] For example, the substrate with a forming surface can be produced according to a preferred embodiment, in general, by the following steps:

[0026] (a) A solution containing at least one compound of the general Formula (I) at a concentration of 0.1%-99% is prepared in a solvent that is organic and polar. Ethyl acetate, acetone, THF, toluene, or xylenes, for example, can be used here as solvents. Mixtures of these solvents are also usable, or they can be supplemented by other solvents. This solution is applied to a substrate that exhibits little if any adhesion to the polymer, such as glass, silicon, various ceramics or other appropriate materials such as polymers (PDMS, Teflon, PMMA, polycarbonate or silicone). The surfaces of the substrates listed can also be chemically modified, for instance, by introducing certain functional groups (—NH₂, —OH, —COOH, —COH, —COOMe, —CF₃, etc.).

[0027] (b) Evaporation of the solvent can proceed without further measures; but in the best case the concentration of the solvent vapor over the substrate is controlled, as are the pressure and the temperature. At the beginning of the first phase of drying, the atmosphere over the coated substrate should be saturated with solvent vapor, with the concentration of the solvent vapor then being reduced slowly over many hours. The temperature can vary from −30° C. to +90° C. The pressure can follow a gradient from normal pressure to water aspirator vacuum (20 Torr) during the first phase of drying. After the first phase of drying, the coated substrate is further dried for a certain period at oil pump vacuum (0.1 Torr).

[0028] The substrate coated with the biocompatible polymer according to Formula (I) can then be used directly, without or after appropriate sterilization. Various coating thicknesses from 0.1 μm to 300 μm or thicker, preferably in the range from 0.5 μm to 30 μm, and especially preferably about 5 μm, are obtained, depending on the concentration of the polymer solution and the conditions used during the first phase of drying.

[0029] Another object of the present invention concerns a substrate with a microstructured surface comprising at least partly a biocompatible polymer according to Formula (I) as defined above, with the size or magnitude of the surface structures being in the range of nanometers, micrometers, or even larger or smaller, preferably in the range of 10 nm to 100 μm. In one preferred embodiment the biocompatible polymer is present on the substrate as a coating with an externally microstructured surface.

[0030] The structuring of the surface is not subject to any particular limitation. For instance, all structures that can be generated photolithographically, with an electron beam, with an ion beam, with a laser, or by other techniques, can be produced. The microstructuring of the surface of the substrate or of the coating can also be obtained by “fusion structuring or melt structuring”, in which a thin wire is brought to the melting temperature of the biocompatible polymer and then melts the desired structure into the surface of the coating by direct contact.

[0031] Special advantages can be attained by means of this structuring with structures that affect the flow behavior of liquids particularly favorably (e.g., sharkskin or lotus effect) imprinted into the surface of the coating or substrate. 

1. Use of a substrate with a forming surface that comprises at least partly a biocompatible polymer with the general formula (I)

in which n stands for 2 to ∞, R¹ to R⁶ are the same or different and stand for an alkoxy, alkylsulfonyl, dialkylamino or aryloxy group or a heterocycloalkyl or heteroaryl group having nitrogen as the hetero atom, as a matrix for producing a biological material implantable in a mammal.
 2. Use according to claim 1, in which the biocompatible polymer is present as a coating on the substrate surface.
 3. Use according to claim 2, in which a layer containing an adhesion promoter is placed between the surface and the coating of biocompatible polymer.
 4. Use according to claim 3, in which the adhesion promoter is a compound containing a polar end group, especially an organosilicon compound.
 5. Use according to claim 4, in which the organosilicon compound is aminopropyltrimethoxysilane.
 6. Use according to one of claims 1 to 5, in which the implantable biological material comprises cells, monolayer or multilayer cellular aggregates, tissue, or cell components.
 7. Substrate with a microstructured surface comprising at least partially a biocompatible polymer with the following general formula (I)

in which n stands for 2 to ∞, R¹ to R⁶ are the same or different and stand for an alkoxy, alkylsulfonyl, dialkylamino or aryloxy group or a heterocycloalkyl or heteroaryl group having nitrogen as the hetero atom, and in which the surface structures have a magnitude on the order of 10 nm to 100 μm.
 8. Substrate according to claim 7, in which the biocompatible polymer is present on the substrate surface as a coating with an external microstructured surface.
 9. Substrate according to claim 8, in which a layer that contains an adhesion promoter is placed between the surface of the substrate and the coating of biocompatible polymer.
 10. Substrate according to claim 9, in which the adhesion promoter is a compound containing a polar end group, especially an organosilicon compound.
 11. Substrate according to claim 10, in which the organosilicon compound is aminopropyltrimethoxysilane. 